Sudden death from acute arrhythmia is a significant risk after myocardial infarction. The risk is great immediately after infarction, and tapers downwardly with the passage of time, although approximately fifty percent of all patients eventually die of ventricular arrhythmia.
It has been determined by numerous investigators that high frequency potentials in the late QRS and ST segments of electrocardiograms is a good indicator of patients at risk of acute arrhythmia. These late potentials have been shown to be characterized by frequencies between about 10 and 250 Hz, and tend to occur in a short time period at the end of the normal QRS segment, with the result that the apparent length of the QRS signal is extended for these patients. However, late potentials are very difficult to detect because of the magnitude of the late potential voltage signals are approximately the same as the typical noise of a raw ECG signal. Detection is further complicated by the intermingling with and close proximity to the QRS segment which is many times greater in amplitude. This problem is further aggravated in patients suffering from bundle branch block which extends the high amplitude portion of the QRS function out into the time period where the late potentials occur.
U.S. Pat. No. 4,422,459, issued to Simson, discloses a method and apparatus for detecting late potentials by signal averaging multiple samples of electrocardiographic signals, typically the conventional X, Y, and Z leads, or the vector sum of these signals to produce a high resolution electrocardiogram (HRECG). A high pass filter is then applied in reverse order to the HRECG. The reverse filtering of latter portion of the QRS segment prevents the ringing artifact which would otherwise result from filtering the high amplitude QRS signal in the forward direction, which would obscure the small amplitude late potentials. The end of the QRS segment, including late potentials, is detected, the RMS of the filtered function over the last 40 milliseconds is computed, and compared to a predetermined RMS value. If the RMS value is less than the predetermined value, it is assumed that late potentials exist, and if the RMS is greater than the predetermined value, it is assumed that no late potentials exist because the 40 millisecond period is actually the end portion of a normal high amplitude QRS section which has a much higher RMS value. Thus, the Simson method is dependent upon the time separation of the late potentials from the main QRS function. Since bundle branch block extends the main QRS event into the region where late potentials occur, this technique cannot be used. Further, the Simson method is not capable of detecting the termination of the normal QRS segment or the independent determination of the onset of the late potentials should they be intermingled with the normal QRS segment.
Because of the limitations on the Simson method, others have attempted to detect late potentials using more direct nonfiltered methods. These efforts have primarily centered around attempts to frequency analyze the HRECG using Fourier analysis by Cain. Initial reports of successful results have not been corroborated by other researchers (Gomes et al, Worley et al, Kelen et al), and there are theoretical reasons why such approaches are not possible as will be more fully developed hereafter.